Tong, L; (2013) Non-Adiabatic Molecular Dynamics And Its Applications In Electron Transport In Nanostructures. Doctoral thesis , UCL (University College London).

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Abstract

Quantum Molecular dynamics (MD) simulations have been widely used to examine the dynamics of interactions between electrons and ions. The most commonly used MD method is the Born-Oppenheimer (BO) approximation, which assumes that the electronic states always stay on the ground-state energy surface of any given ionic configuration during the course of the simulation. The BO approximation, however, is not appropriate for studies of systems that are out of equilibrium, such as electron transport processes. One way of allowing an MD simulation to explore the non-equilibrium and excited states is to use the Ehrenfest approximation, which gives a full time-dependent quantum mechanical treatment of electrons, while regarding the ions as classical particles. This thesis gives a careful derivation of the equations of motion (EoM) in Ehrenfest dynamics, in the context of the Time-Dependent Density Functional Theory represented in a non- orthogonal and incomplete basis centered on the moving ions. The EoM were implemented in an existing ab initio electronic structure code Plato. Various propagators for solving the electronic EoM were studied and compared. A micro- canonical model based on the Ehrenfest MD for simulating electron transport processes has been developed. Extensive real-time transport studies were performed on aromatic hydrocarbon compounds attached to graphene nanoribbon leads. A self-consistent non-orthogonal tight-binding model was used to enable large-scale simulations with reasonable computational cost. The quasi-steady-state currents together with the current induced dynamical effects were measured from the simulations and analysed. The quasi-steady-state currents were compared with the steady-state solutions obtained from a time-independent non-equilibrium Green functions method commonly used by the electron transport community.

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